Many potential neural prostheses, including visual, auditory, and motor prostheses, will not be feasible until microelectrode arrays are developed that allow multiple, small clusters of neurons to be independently stimulated. This project will involve research and development on thin-film microelectrode arrays capable of independently stimulating as many as 512 such small clusters of cells. Specifically, these microelectrode arrays are being designed to provide microstimulation at multiple sites in the visual cortex, the cochlear nucleus and the lumbrosacral spinal cord. Micromachining of silicon, combined with integration of electronic circuits on the micromachined structure, permits fabrication of active circuit microelectrodes with multiple stimulating sites on multiple shanks. Microelectrode arrays currently under development have 64 stimulation sites placed along 8 or 16 penetrating shanks. These 64-site, two-dimensional microelectrode arrays can be assembled into a 3-dimensional array with 512 stimulating sites. These thin- film stimulating microelectrodes have several advantages over more conventional wire bundle microelectrodes for multiple site, highly selective stimulation. Their stimulating site density is at least an order of magnitude greater than wire bundle electrodes and permits stimulation site spacing with dimensions comparable to the dimensions of neurons. The designs provide circuitry which permits extracorporeally generated stimulus instructions for many neural stimulating sites to be combined into a single signal and then decoded by integrated electronics on the implant. The integrated electronics also permit the arrays to be designed with integrated telemetry, eliminating the need for tethering cables.